Astronomers write a recipe for other Earths

Scientists use instrument to measure chemical composition

Twenty years after the first confirmed discovery of an exoplanet — a planet orbiting a major star, but not our own — these celestial bodies have really hit their stride. Last year broke records, with astronomers discovering over 800 alien planets in 2014 alone. But now that finding a new exoplanet has become old hat, astronomers are focusing on getting to know these alien worlds intimately, and learning how they form.

Earth is what is called a rocky planet, as opposed to being made up of gas or ice. Scientists now know that planets of the right size to be Earth-like are fairly common in the galaxy. But do these other rocky planets follow the same basic recipe as our own? According to new research presented Monday at a meeting of the American Astronomical Society, the answer is yes.

Scientists used HARPS-North, an instrument that can measure the masses of Earth-sized planets. And when it comes to studying a faraway planet, mass and density are the best way to estimate chemical composition.

When they compared known exoplanets, the researchers found that not all Earth-size planets were Earth-like. Some of them have very low densities relative to Earth. But in general, density went up as mass went down. This is in line with what we already know — that larger planets are usually made of gas and ice, while small ones are more likely to be dense solids.

But when planets had the right density for rockiness, the researchers found, they also seemed to have the same chemical composition as Earth.

Tracking down these rocky planets, and figuring out which have liquid water, could (at least for now) be scientists’ best bet at finding life as we know it.

Another study presented at the same conference this week in Seattle suggests that the oldest super-Earths may be the most likely to harbor life. Laura Schaefer, of the Harvard-Smithsonian Center for Astrophysics, used computer simulations to estimate how water might form and behave on a planet 1.5 times the size of Earth and found that (based on the evolutionary history of our own planet) life might have a slower start.

So on a super-Earth sized planet, she believes, it might take 5.5 billion years to cook up complex life — which is about a billion years longer than it took on Earth.

Commenting on the day’s presentations during a news conference Monday, Yale professor Debra Fischer reminded attendees that the first exoplanet was only discovered 20 years ago. Until then, astronomers worried that there might not be any possibility of life outside of Earth.

“Here’s the universe twinkling with the lights of other stars, and we had to think that maybe there weren’t other planets orbiting around them.” Fischer said.

But now we know that almost every star has its own planetary system — a remarkable shift for just two decades of work.

“And now we’re talking about a planet when we can barely see the star,” Fischer said, “and yet we’re able to talk about the mass of the planet, we can do a diagram slicing through to see the core of the planet, we can have a serious scientific dialogue. This is incredibly moving.”

The next questions to answer are where the habitable planets around nearby stars are, and whether they harbor life. Then we’ll need to figure out how to get to them. But that’s not far off, Fischer said Monday — in fact, we could be there within the next 50 years.